In synchronous reluctance permanent magnet motors, interior permanent magnet motors, and surface permanent magnet motors, the rotor often becomes the hottest part of the motor during operation as it is the most difficult to effectively cool. The excessive temperature tends to degrade magnet performance and motor efficiency. Often in these types of motors, excessive heat is transferred from the rotor to the stator through the gap between the rotor and the stator. Thus, in addition to an increase of rotor temperature, stator temperature also increases. While liquid coolants, such as oil, may be used in the interior of the motor to cool the internal motor components, traditional methods of cooling with liquid coolants typically result in prohibitively high friction and windage losses. For instance, the internal volume of the motor may be filled with oil to cool internal motor components, and a pump may be used to circulate the oil through the motor. Filling the internal volume of the motor with oil, typically creates high friction and windage losses.
These disadvantages may be overcome. Substantial cooling of electrical machines may be achieved by directing a mixture of gas and cooling liquid or coolant, (for instance, oil and air) through the gap between the rotor and stator of the electro-dynamic machine. The amount of coolant is controlled to maintain friction and windage losses at an acceptable level. The gas may be pressurized to generate the flow of the gas and coolant mixture through the gap. As will be further discussed in great detail below, the methods and constructions of the electro-dynamic machine enable the gas-coolant mixture to be directed to the gap between a rotor and stator, thereby putting the rotor in close contact with the gas-coolant mixture for enhanced cooling of the electrical machine.